Particles ranging in size from 1 nm (nanometer, 10.sup.-9 meter) to more than 100 nm exhibit unique and useful surface and interface properties because they contain a high proportion of surface-to-bulk atoms. Uses of these particles include but are not limited to heterogenous catalysts, ceramic materials fabrication, intermetallics, electronics semiconductor fabrication, magnetic recording media, and superconductors.
Production of nanometer-sized particles is currently accomplished in a variety of ways, including gas phase condensation; laser synthesis processes; freeze drying methods; flame or plasma torch reactions; vacuum synthesis methods utilizing sputtering, laser ablation, liquid metal ion sources; reverse micelle solutions; solidification from the liquid state; and hydrothermal methods.
It is a long-felt need in the art of nanometer-sized particle production to be able to produce larger quantities at faster rates and to be able to control product particle size distribution in order to improve performance and cost of products, including but not limited to those enumerated above.
Because a preferred embodiment of the present invention is a hydrothermal method, hydrothermal methods are further summarized herein. Hydrothermal methods utilize aqueous solutions at conditions of elevated temperatures and/or elevated pressures wherein particles are formed by nucleation and grown under these conditions to produce powder products.
Conventional hydrothermal methods begin with making a solution of a soluble precursor in a water based solvent, or aqueous slurry of insoluble or partially soluble solids. The batch is placed in a vessel. Particles are formed by chemical reactions resulting in nucleation forming precipitates within the vessel. Reactions may be enhanced by heating, or pressurization, or both. Heating includes a "ramped" heating stage to bring the solution to a desired temperature.
Hydrothermal methods have been carried out in batch, semi-batch, and continuous processes. Whether batch, semi-batch, or continuous process, these methods are characterized by reactor residence times from about 10 min. to well over several hours, and even days, to achieve 90 percent conversion of starting material to precipitated product.
Hydrothermal reactions are carried out at elevated temperature and pressure, generally for the purpose of obtaining specific crystalline structure useful as a catalyst, pigment, or other purpose. Often materials known as mineralizers are added to modify the solid solubilities in the solution, thereby modifying rate of particle growth for the purpose of controlling the specific crystalline structure.
Particle sizes obtained using hydrothermal processing methods are a result of concentrations of reactants, concentrations and type of mineralizers, the amount of time that the reactants are in contact with the hydrothermal solution, and the temperature and pressure of the reactant solution. Using current methods, it is difficult to control the reactants' contact time at given conditions of temperature and pressure because of large total heat capacity of vessels and solutions.
Hydrothermally formed particles include but are not limited to oxides and hydroxides formed by hydrolysis or oxidation reactions in aqueous solvent systems. More specifically, particle products include but are not limited to iron oxide, titanium oxide, nickel oxide, zirconium oxide, aluminum oxide, and silicon oxide. Precursor solutions from which particles are made include but are not limited to aqueous nitrate solutions, sulfate solutions, and oxalate solutions. For example, iron oxide particles may be made from Fe(NO.sub.3).sub.3 or Fe(NH.sub.4)(SO.sub.4).sub.2.
Other materials can be formed by reactions in non-aqueous solvent systems; for example, organometallic species as well as non-oxide ceramic particles, formed by reaction of a precursor with a solvent.
Further operational details of hydrothermal methods may be found in Hydrothermal Synthesis of Advanced Ceramic Powders, William J. Dawson, Ceram. Bull., 67, 1988, pp. 1673-1677, and in The Role of Hydrothermal Synthesis in Preparative Chemistry, Albert Rabenau, Agnew. Chem. Int. Ed. Engl., 24, 1985, pp. 1026-1040.
Another example of preparing fine powders is found in U.S. Pat. No. 4,734,451, issued on Mar. 29, 1988, to R. D. Smith, entitled SUPERCRITICAL FLUID MOLECULAR SPRAY THIN FILMS AND FINE POWDERS. Smith teaches the formation of fine powders by dissolving a solid material into a supercritical fluid solution and rapidly expanding the solution through a short orifice into a region of low pressure, thereby nucleating and forming particles in the region of low pressure. This process differs from the ones described above inasmuch as it is a continuous process and there is no chemical reaction between the solid material and the supercritical fluid solution. While the Smith process is useful for soluble polymers, organic compounds, and many inorganic compounds, it is not useful for insoluble or substantially insoluble ceramic materials, metal oxides, and other above-mentioned substantially insoluble materials. In addition to requiring dissolution of the particle forming compound, the Smith process requires operations at conditions under which carrier solutions have no liquid droplet formation upon expansion to low pressure, whereas the present invention does not require this limitation.
Presently, sulfated metal oxide catalysts are made by essentially dipping or otherwise imparting a coated layer of catalytically active material onto a carrier material (Materials Chemistry and Physics, 26 (1990) 213-237). Significantly, the sulfation step occurs after crystallite formation of the carrier material. The significance of the sulfation step occurring after the crystallite formation of the carrier material is that the sulfation is initially deposited primarily on a surface portion of the carrier material. The carrier material may be ceramic or metal oxide, and the catalytically active material is a sulfate in combination with the carrier material. Where the carrier material is a metal oxide, the metal oxide is selected from the group including but not limited to zirconium oxide, titanium oxide, hafnium oxide, iron oxide and tin oxide. Additionally, the catalytically active material may include a promoter of a metal including but not limited to nickel, copper, iron, and manganese, or any combination thereof. In use, carbon deposits form on the catalyst particle surfaces inhibiting the catalytic activity and ultimately requiring removal of the carbon in order to regenerate the catalyst active sites. However, because most of the catalytically active material is on the surface of the catalyst particles, removal of the carbon deposits also removes a portion of the active catalyst material, thereby limiting the number of times that removal of carbon deposits restores the catalyst particle to a useful level of catalytic activity.